Impact and wear resistant component, and method for producing the same

ABSTRACT

A ripper shank as the impact and wear resistant component is made of a steel of a specific component composition which has a hardness of HRC 53 or more and HRC 57 or less. The steel includes a matrix including a martensite phase and a residual austenite phase, and first nonmetallic particles dispersed in the matrix and including at least one species selected from the group consisting of MnS, TiCN, and NbCN. The steel does not include a M23C6 carbide.

TECHNICAL FIELD

The present invention relates to a component (impact and wear resistantcomponent) that is subjected to repeated impact and wears by contactwith earth and sand, such as a ground engaging tool (hereinafter, GET)component used in construction or mining equipment, and to a method forproducing the same.

This application claims priority based on Japanese Patent ApplicationNo. 2018-243881 filed on Dec. 27, 2018, the entire contents of which areincorporated herein by reference.

BACKGROUND ART

A ripper device is a rear attachment of a work vehicle such as abulldozer, and is used to scrape up earth, sand, and bedrock. Rippingwork can be performed as the work machine is advanced with a ripperpoint attached to the distal end of the ripper shank being penetratedinto the ground. While the ripper shank is a strength member of theripper device, it is an impact and wear resistant component that sufferswear and deformation. Although SCrB steel, JIS SNCM431H steel, etc. haveconventionally been used as the steel material constituting the rippershank, a material having even better durability is desired.

To improve the durability of an impact and wear resistant component, itis necessary to impart high wear resistance and high proof stress(strength) to the material constituting the component. Simply increasingthe strength of a component, however, leads to reduction in toughness ofthe material constituting the component. The surface of the componentmay crack or the component may break, giving rise to the need forreplacement of the component. As such, in order to improve thedurability of the impact and wear resistant component, it is necessaryto maintain ductility (toughness) at a high level while achieving highproof stress (strength) of the material.

As a steel material constituting a component of construction equipment,a high-toughness and wear-resistant steel having excellent durabilityhas been proposed (see, for example, Japanese Patent ApplicationLaid-Open No. S61-166954 (Patent Literature 1)). Further, as a steel fora tracked undercarriage component, a steel containing about 0.4 mass %carbon and various alloy elements added therein has been proposed (see,for example, Japanese Translation of PCT International Publication No.2014/185337 (Patent Literature 2)).

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open No.S61-166954

Patent Literature 2: Japanese Translation of PCT InternationalPublication No. 2014/185337

SUMMARY OF INVENTION Technical Problem

When an impact and wear resistant component, particularly a GETcomponent, is produced using the steel disclosed in Patent Literature 1or 2, the resultant component will have a high strength. Further, asteel having an improved 0.2% proof stress will be able to, for example,suppress deformation (plastic flow) of the contact surface with theripper point in the ripper shank. However, when the steel materialdisclosed in Patent Literature 1 is used to produce a large ripper shankhaving a wall thickness of 100 mm and a mass of about 1 ton, forexample, the component will suffer a decrease in strength (insufficienthardenability) at the center in its wall thickness. Further, a componentproduced using the steel disclosed in Patent Literature 2 through acommon production process tends to exhibit a small reduction of area ina tensile test. According to the investigations conducted by the presentinventors, the smaller reduction of area in the tensile test leads tolower resistance to breakage. That is, further improvement in durabilityis desired for the impact and wear resistant component produced througha common production process using the steel disclosed in PatentLiterature 2.

One of the objects of the present invention is to provide an impact andwear resistant component excellent in durability and a method forproducing the same.

Solution to Problem

An impact and wear resistant component according to the presentinvention is made of a steel containing not less than 0.41 mass % andnot more than 0.44 mass % C, not less than 0.2 mass % and not more than0.5 mass % Si, not less than 0.2 mass % and not more than 1.5 mass % Mn,not less than 0.0005 mass % and not more than 0.0050 mass % S, not lessthan 0.6 mass % and not more than 2.0 mass % Ni, not less than 0.7 mass% and not more than 1.5 mass % Cr, not less than 0.1 mass % and not morethan 0.6 mass % Mo, not less than 0.02 mass % and not more than 0.03mass % Nb, not less than 0.01 mass % and not more than 0.04 mass % Ti,not less than 0.0005 mass % and not more than 0.0030 mass % B, and notless than 20 mass ppm and not more than 60 mass ppm N, with the balanceconsisting of iron and unavoidable impurities, and having a hardness ofHRC 53 or more and HRC 57 or less. The steel includes a matrix includinga martensite phase and a residual austenite phase, and first nonmetallicparticles dispersed in the matrix and including at least one speciesselected from the group consisting of MnS, TiCN, and NbCN. The steeldoes not include a carbide represented as M₂₃C₆ (where M represents themetallic elements constituting the steel).

A method for producing an impact and wear resistant component accordingto the present invention includes the steps of: preparing a steelmaterial made of a steel containing not less than 0.41 mass % and notmore than 0.44 mass % C, not less than 0.2 mass % and not more than 0.5mass % Si, not less than 0.2 mass % and not more than 1.5 mass % Mn, notless than 0.0005 mass % and not more than 0.0050 mass % S, not less than0.6 mass % and not more than 2.0 mass % Ni, not less than 0.7 mass % andnot more than 1.5 mass % Cr, not less than 0.1 mass % and not more than0.6 mass % Mo, not less than 0.02 mass % and not more than 0.03 mass %Nb, not less than 0.01 mass % and not more than 0.04 mass % Ti, not lessthan 0.0005 mass % and not more than 0.0030 mass % B, and not less than20 mass ppm and not more than 60 mass ppm N, with the balance consistingof iron and unavoidable impurities; hot forging or hot rolling the steelmaterial to obtain a formed body; performing normalizing treatment on anentirety of the formed body by cooling the formed body from atemperature not lower than 945° C. and not higher than 1000° C. to atemperature not higher than a temperature corresponding to the M_(s)point of the steel; and performing quench hardening treatment on theformed body having undergone the normalizing treatment and, thereafter,adjusting a hardness of the formed body to be HRC 53 or more and HRC 57or less by heating the formed body to a temperature not lower than 150°C. and not higher than 250° C.

Effects of the Invention

According to the impact and wear resistant component and its producingmethod described above, it is possible to provide an impact and wearresistant component excellent in durability and a method for producingthe same.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing the structure of a ripper deviceincluding a ripper shank and a ripper point;

FIG. 2 is a schematic perspective view showing the state of connectionbetween the ripper shank and the ripper point;

FIG. 3 is a schematic cross-sectional view showing the structure of theripper shank;

FIG. 4 is a flowchart schematically illustrating the steps of producinga ripper shank;

FIG. 5 shows optical micrographs of a microstructure of the steel;

FIG. 6 shows SEM photographs of nonmetallic particles;

FIG. 7 shows observation results using an optical microscope and SEM,and elemental mapping results;

FIG. 8 shows a result of identification of a product present at a grainboundary; and

FIG. 9 shows a relationship between heating temperature and reduction ofarea.

DESCRIPTION OF EMBODIMENT Outline of Embodiment

An impact and wear resistant component of the present application ismade of a steel containing not less than 0.41 mass % and not more than0.44 mass % C, not less than 0.2 mass % and not more than 0.5 mass % Si,not less than 0.2 mass % and not more than 1.5 mass % Mn, not less than0.0005 mass % and not more than 0.0050 mass % S, not less than 0.6 mass% and not more than 2.0 mass % Ni, not less than 0.7 mass % and not morethan 1.5 mass % Cr, not less than 0.1 mass % and not more than 0.6 mass% Mo, not less than 0.02 mass % and not more than 0.03 mass % Nb, notless than 0.01 mass % and not more than 0.04 mass % Ti, not less than0.0005 mass % and not more than 0.0030 mass % B, and not less than 20mass ppm and not more than 60 mass ppm N, with the balance consisting ofiron and unavoidable impurities, and having a hardness of HRC 53 or moreand HRC 57 or less. The steel includes a matrix including a martensitephase and a residual austenite phase, and first nonmetallic particlesdispersed in the matrix and including at least one species selected fromthe group consisting of MnS, TiCN, and NbCN. The steel does not includea carbide represented as M₂₃C₆ (where M represents the metallic elementsconstituting the steel).

In the impact and wear resistant component described above, the steelmay further contain at least one species selected from the groupconsisting of not less than 0.05 mass % and not more than 0.20 mass % V,not less than 0.01 mass % and not more than 0.15 mass % Zr, and not lessthan 0.1 mass % and not more than 2.0 mass % Co.

Firstly, a description will be made about the reasons for limiting thecomponent composition of the steel constituting the impact and wearresistant component of the present application to the above-describedranges.

Carbon (C): Not Less than 0.41 Mass % and Not More than 0.44 Mass %

Carbon is an element that greatly affects the hardness of the steel. Ifthe carbon content is less than 0.41 mass %, it will be difficult toobtain a hardness of HRC 53 or more in a portion having a wall thicknessof about 100 mm, for example, with quenching and tempering. On the otherhand, the carbon content exceeding 0.44 mass % will decrease thereduction of area and reduce the breakage resistance. The carbon contentis thus necessary to be within the above-described range. From thestandpoint of readily securing a sufficient hardness, the carbon contentis preferably 0.42 mass % or more.

Silicon (Si): Not Less Than 0.2 Mass % and Not More Than 0.5 Mass %

Silicon is an element that has the effects of improving thehardenability of the steel, enhancing the matrix of the steel, andimproving the resistance to temper softening, and also has a deoxidizingeffect in the steelmaking process. If the silicon content is 0.2 mass %or less, the above effects cannot be obtained sufficiently. If thesilicon content exceeds 0.5 mass %, however, the reduction of area tendsto decrease. The silicon content is thus necessary to be within theabove-described range.

Manganese (Mn): Not Less Than 0.2 Mass % and Not More Than 1.5 Mass %

Manganese is an element effective in improving the hardenability of thesteel, and also having a deoxidizing effect in the steelmaking process.If the manganese content is 0.2 mass % or less, the above effects cannotbe obtained sufficiently. If the manganese content exceeds 1.5 mass %,however, the hardness before quench hardening will increase, leading todegradation in workability. From the standpoint of securing sufficienthardenability of the steel, the manganese content is preferably 0.4 mass% or more. Further, focusing on the workability, the manganese contentis preferably 0.9 mass % or less, and more preferably 0.8 mass % orless.

Sulfur (S): Not Less Than 0.0005 Mass % and Not More Than 0.0050 Mass %

Sulfur is an element that improves the machinability of the steel.Sulfur is also an element that is mixed during the steelmaking processeven if not added intentionally. If the sulfur content is less than0.0005 mass %, the machinability will decrease, and the production costof the steel will increase. On the other hand, according to theinvestigations of the present inventors, in the component composition ofthe steel of the present application, the sulfur content greatly affectsthe reduction of area. If the sulfur content exceeds 0.0050 mass %, thereduction of area will decrease, making it difficult to obtainsufficient breakage resistance. The sulfur content is thus necessary tobe within the above-described range. The sulfur content of 0.0040 mass %or less can further improve the breakage resistance.

Nickel (Ni): Not Less Than 0.6 Mass % and Not More Than 2.0 Mass %

Nickel is an effective element in improving the toughness of the matrixof the steel. If the nickel content is less than 0.6 mass %, such aneffect cannot be exerted sufficiently. If the nickel content exceeds 2.0mass %, however, nickel becomes more likely to segregate in the steel.This may cause variation in the mechanical properties of the steel. Thenickel content is thus necessary to be within the above-described range.Further, with the nickel content exceeding 1.5 mass %, the improvementin toughness will become moderate, and the production cost of the steelwill increase. From these standpoints, the nickel content is preferably1.5 mass % or less. On the other hand, in the case of a steel having ahardness of HRC 53 or more, in order to sufficiently exert the effect ofimproving the toughness of the matrix of the steel, the nickel contentis preferably 1.0 mass % or more.

Chromium (Cr): Not Less Than 0.7 Mass % and Not More Than 1.5 Mass %

Chromium improves the hardenability of the steel and also enhances theresistance to temper softening. In particular, chromium being added incombination with molybdenum, niobium, vanadium, and the likeconsiderably enhances the resistance to temper softening of the steel.If the chromium content is less than 0.7 mass %, the above effectscannot be exerted sufficiently. If the chromium content exceeds 1.5 mass%, however, the improvement of the resistance to temper softening willbecome moderate, and the production cost of the steel will increase. Thechromium content is thus necessary to be within the above-describedrange.

Molybdenum (Mo): Not Less Than 0.1 Mass % and Not More Than 0.6 Mass %

Molybdenum improves the hardenability of the steel and enhances theresistance to temper softening. Molybdenum also has the function ofimproving the high temperature tempering brittleness. If the molybdenumcontent is less than 0.1 mass %, the above effects cannot be exertedsufficiently. If the molybdenum content exceeds 0.6 mass %, however, theabove effects will be saturated. The molybdenum content is thusnecessary to be within the above-described range.

Niobium (Nb): Not Less Than 0.02 Mass % and Not More Than 0.03 Mass %

Niobium is effective in improving the strength and toughness of thesteel. In particular, niobium is a highly effective element in improvingthe toughness because it makes the crystal grains of the steel extremelyfine when added in combination with chromium and molybdenum. To securesuch effects, the niobium content should be 0.02 mass % or more. If theniobium content exceeds 0.03 mass %, however, the crystallization ofcoarse eutectic NbC and the formation of a large amount of NbC cause adecrease in the amount of carbon in the matrix, leading to degradationin strength and toughness of the steel. Further, the niobium contentexceeding 0.03 mass % will increase the production cost of the steel.The niobium content is thus necessary to be within the above-describedrange.

Titanium (Ti): Not Less Than 0.01 Mass % and Not More Than 0.04 Mass %

Titanium is effective in improving the toughness of the steel. Further,the addition of Ti can form Ti(C,N) and refine the crystal grains of thesteel. If the titanium content is less than 0.01 mass %, such effectsare small. If the titanium content exceeds 0.04 mass %, however, thetoughness of the steel may rather deteriorate. The titanium content isthus necessary to be within the above-described range.

Boron (B): Not Less Than 0.0005 Mass % and Not More Than 0.0030 Mass %

Boron is an element that considerably improves the hardenability of thesteel. The addition of boron can decrease the addition amounts of theother elements added for the purpose of improving the hardenability, andcan reduce the production cost of the steel. As compared to phosphorus(P) and sulfur, boron is more likely to segregate in the prior austenitegrain boundary, and it particularly expels sulfur from the grainboundary, thereby improving the grain boundary strength. If the boroncontent is 0.0005 mass % or less, the above effects cannot be exertedsufficiently. The boron content exceeding 0.0030 mass %, however, maydecrease the toughness of the steel. The boron content is thus necessaryto be within the above-described range.

Nitrogen (N): Not Less Than 20 Mass ppm and Not More Than 60 Mass ppm

Nitrogen may deteriorate the toughness of the steel, except the casewhere nitrogen together with carbon forms carbonitrides with Ti or Nb torefine the crystal grains. The nitrogen content is thus necessary to be60 mass ppm or less. The nitrogen content of less than 20 mass ppm,however, will increase the production cost of the steel. The nitrogencontent is thus necessary to be within the above-described range.

Vanadium (V): Not Less Than 0.05 Mass % and Not More Than 0.20 Mass %

Vanadium is not an indispensable element. Vanadium, however, forms finecarbides, contributing to the refinement of crystal grains. If thevanadium content is less than 0.05 mass %, the above effect cannot beobtained sufficiently. If the vanadium content exceeds 0.20 mass %,however, the above effect will be saturated. Vanadium is a relativelyexpensive element, so it is preferably added in a minimum requiredamount. Thus, in the case of adding vanadium, the addition amount withinthe above-described range is appropriate.

Zirconium (Zr): Not Less Than 0.01 Mass % and Not More Than 0.15 Mass %

Zirconium is not an indispensable element, but it has the effect offurther improving the toughness of the steel by making carbides in theform of fine spherical particles dispersed in the steel. If thezirconium content is less than 0.01 mass %, its effect cannot beobtained sufficiently. If the zirconium content exceeds 0.15 mass %,however, the toughness of the steel may rather deteriorate. Thus, in thecase of adding zirconium, the addition amount within the above-describedrange is appropriate.

Cobalt (Co): Not Less Than 0.1 Mass % and Not More Than 2.0 Mass %

Cobalt is not an indispensable element, but it increases the solidsolubility of chromium, molybdenum, and other carbide-forming elementsto the matrix, and also improves the resistance to temper softening ofthe steel. The addition of cobalt thus achieves finer carbides and ahigher tempering temperature, thereby improving the strength andtoughness of the steel. If the cobalt content is less than 0.1 mass %,the above effects cannot be obtained sufficiently. On the other hand,because of its expensiveness, cobalt added in a large amount willincrease the production cost of the steel. These problems becomeprominent with a cobalt content exceeding 2.0 mass %. Thus, in the caseof adding cobalt, the addition amount within the above-described rangeis appropriate.

Unavoidable Impurities

Besides the components intentionally added during the productionprocess, elements other than those described above may be mixed into thesteel as unavoidable impurities. Phosphorus (P) as an unavoidableimpurity is preferably contained in an amount of 0.010 mass % or less.Copper (Cu) as an unavoidable impurity is contained in an amount ofpreferably 0.1 mass % or less and more preferably 0.05 mass % or less.Aluminum (Al) as an unavoidable impurity is contained in an amount ofpreferably 0.04 mass % or less.

The impact and wear resistant component of the present application ismade of a steel having the above-described appropriate componentcomposition. Further, in the impact and wear resistant component of thepresent application, the steel constituting the impact and wearresistant component does not include a carbide represented as M₂₃C₆(where M represents the metallic elements constituting the steel, mainlyat least one of Cr and Mo; hereinafter, referred to as “M₂₃C₆ carbide”).

According to the investigations conducted by the present inventors, inthe case of adopting a steel having the above-described appropriatecomponent composition as the steel constituting an impact and wearresistant component, when the component is produced with a commonproduction process, M₂₃C₆ carbides are generated at the grain boundariesof the steel. With the M₂₃C₆ carbides generated, the Cr and Mo contentsdecrease in the region around the M₂₃C₆ carbides. The hardenability inthe region thus decreases, and a bainite structure is formed. That thesteel contains not only a martensite structure, but also brittle M₂₃C₆carbides at the grain boundaries as well as brittle bainite structurenear the grain boundaries attributable thereto, results in a smallerreduction of area in the tensile test of the steel. A lower reduction ofarea of the steel leads to a reduced breakage resistance of the impactand wear resistant component made of the steel.

As a result of investigating the way of improving the durability of theimpact and wear resistant components, the present inventors haveobtained findings that adopting a steel having the above-describedappropriate component composition and eliminating the M₂₃C₆ carbidesfrom the steel structure can obtain an impact and wear resistantcomponent improved in breakage resistance and excellent in durability.In the impact and wear resistant component of the present application,the steel having the above-described appropriate component compositionis adopted as the steel constituting the impact and wear resistantcomponent, and no M₂₃C₆ carbides are included in the steel structure.The impact and wear resistant component of the present application isthus an impact and wear resistant component excellent in durability.

In the present application, the state where the steel includes no M₂₃C₆carbides means a state where M₂₃C₆ carbides are not found when the crosssection of the impact and wear resistant component is observed using afield-emission scanning electron microscope (FE-SEM) and an area of 80μm² including the grain boundary of the steel is examined for 10 or morefields of view. The M₂₃C₆ carbide can be identified, when a possibleproduct of M₂₃C₆ carbide is found for example in the above-describedmanner, by detecting the product in a bright-field image of a scanningtransmission electron microscope (STEM) and then confirming the selectedarea diffraction (SAD) pattern of the product.

In the impact and wear resistant component described above, the matrixmay have a grain size number of 5 or more and 8 or less. With thisconfiguration, excellent toughness can readily be imparted to the impactand wear resistant component.

In the impact and wear resistant component described above, themartensite phase constituting the matrix may be a lowtemperature-tempered martensite phase. With this configuration,excellent toughness can readily be imparted to the impact and wearresistant component.

As used herein, the low temperature-tempered martensite phase means aphase made up of a structure (obtained through low temperaturetempering) which is obtained when a steel that has been quenched istempered at a temperature not lower than 150° C. and not higher than250° C. The phase being the low temperature-tempered martensite phasecan be confirmed through investigation of the hardness, carbideprecipitation state, etc. of the phase.

A method for producing an impact and wear resistant component of thepresent application includes the steps of: preparing a steel materialmade of a steel containing not less than 0.41 mass % and not more than0.44 mass % C, not less than 0.2 mass % and not more than 0.5 mass % Si,not less than 0.2 mass % and not more than 1.5 mass % Mn, not less than0.0005 mass % and not more than 0.0050 mass % S, not less than 0.6 mass% and not more than 2.0 mass % Ni, not less than 0.7 mass % and not morethan 1.5 mass % Cr, not less than 0.1 mass % and not more than 0.6 mass% Mo, not less than 0.02 mass % and not more than 0.03 mass % Nb, notless than 0.01 mass % and not more than 0.04 mass % Ti, not less than0.0005 mass % and not more than 0.0030 mass % B, and not less than 20mass ppm and not more than 60 mass ppm N, with the balance consisting ofiron and unavoidable impurities; hot forging or hot rolling the steelmaterial to obtain a formed body; performing normalizing treatment on anentirety of the formed body by cooling the formed body from atemperature not lower than 945° C. and not higher than 1000° C. to atemperature not higher than a temperature corresponding to the M_(s)point of the steel; and performing quench hardening treatment on theformed body having undergone the normalizing treatment and, thereafter,adjusting a hardness of the formed body to be HRC 53 or more and HRC 57or less by heating the formed body to a temperature not lower than 150°C. and not higher than 250° C.

In the impact and wear resistant component producing method describedabove, the steel may further contain at least one species selected fromthe group consisting of not less than 0.05 mass % and not more than 0.20mass % V, not less than 0.01 mass % and not more than 0.15 mass % Zr,and not less than 0.1 mass % and not more than 2.0 mass % Co.

In the impact and wear resistant component producing method of thepresent application, after a steel material made of the steel having theabove-described appropriate component composition is prepared, the steelmaterial is hot forged or hot rolled to obtain a formed body. In thecooling process following the hot forging or hot rolling, M₂₃C₆ carbidesare generated at the grain boundaries of the steel. Thereafter, in theimpact and wear resistant component producing method of the presentapplication, normalizing treatment is performed on the entirety of theformed body in which the formed body is cooled from a temperature notlower than 945° C. and not higher than 1000° C. to a temperature nothigher than the temperature corresponding to the M_(s) point of thesteel. With the normalizing treatment of heating to a temperature rangeof not lower than 945° C. and then cooling being performed, the M₂₃C₆carbides previously generated dissolve into the matrix of the steel anddisappear. Thereafter, quench hardening treatment is performed and thenthe formed body is heated to a temperature not lower than 150° C. andnot higher than 250° C. to adjust the hardness of the steel to be HRC 53or more and HRC 57 or less. In this manner, it is readily possible toproduce the impact and wear resistant component of the presentapplication that is made of the steel including no M₂₃C₆ carbides.

Specific Example of Embodiment

An embodiment of the impact and wear resistant component of the presentinvention will be described below with reference to the drawings. In thefollowing drawings, the same or corresponding parts are denoted by thesame reference numerals, and the description thereof will not berepeated.

Firstly, referring to FIGS. 1 to 3, a ripper shank as an impact and wearresistant component in the present embodiment will be described. FIG. 1is a schematic view showing the structure of a ripper device including aripper shank and a ripper point. FIG. 2 is an exploded perspective viewof the ripper shank and the ripper point. FIG. 3 is a schematiccross-sectional view showing the structure of the ripper shank.

Referring to FIG. 1, the ripper device 1 of the present embodiment is,for example, a ripper device attached to a bulldozer. The ripper device1 is attached to the rear (opposite the side on which a blade (soilremoval plate) is disposed) of the vehicle body of the bulldozer. Theripper device 1 includes an arm 31, a lift cylinder 32, a tilt cylinder33, a ripper support member 34, a ripper shank 10, and a ripper point20.

The arm 31 has a rod shape. The arm 31 has one end connected to abracket (not shown) mounted on the vehicle body of the bulldozer, andthe other end connected to the ripper support member 34. The rippersupport member 34 is pivotably connected to the other end of the arm 31.

The lift cylinder 32 and the tilt cylinder 33 have their one endsconnected to the bracket (not shown) mounted on the vehicle body of thebulldozer. The lift cylinder 32 and the tilt cylinder 33 have theirother ends connected to the ripper support member 34. The lift cylinder32 and the tilt cylinder 33 are hydraulic cylinders that can be extendedand contracted in the longitudinal direction. The ripper support member34 is pivotably connected to the other ends of the lift cylinder 32 andthe tilt cylinder 33. Of the ripper support member 34, the regionconnected to the lift cylinder 32 is located between the regionconnected to the arm 31 and the region connected to the tilt cylinder33.

Referring to FIGS. 1 and 2, the ripper shank 10 is made of steel. Theripper shank 10 includes a distal end 15 as one end and a proximal end14 as the other end in the longitudinal direction. The region includingthe distal end of the ripper shank 10 is bent toward the sideapproaching the vehicle body of the bulldozer. The region of the rippershank 10 between its distal end 15 and proximal end 14 is supported bythe ripper support member 34. The ripper point 20 is attached to thedistal end 15 of the ripper shank 10. Of the ripper support member 34,the region connected to the arm 31 is positioned closer to the ripperpoint 20 as compared to the region connected to the tilt cylinder 33 andthe region connected to the lift cylinder 32.

In the ripper device 1, the extension and contraction of the liftcylinder 32 cause the ripper shank 10 to move up and down. The extensionand contraction of the tilt cylinder 33 cause the ripper shank 10 totilt. With the ripper shank 10 in a lowered state and tilted to causethe ripper point 20 to penetrate the ground the vehicle body of thebulldozer is advanced, whereby earth, sand, and bedrock are scraped up.

Referring to FIGS. 1 to 3, the ripper shank 10 has a through hole, aripper shank through hole 11, formed therein. The ripper point 20 has athrough hole, a ripper point through hole 25, formed therein. In thestate where the ripper point 20 is attached to the ripper shank 10, theripper point through hole 25 and the ripper shank through hole 11 form acontinuous through hole. A pin 51 inserted into the continuous throughhole secures the ripper point 20 to the ripper shank 10.

Referring to FIG. 3, the ripper point 20 has a recess 22 formed torecess from its proximal end 23 side toward its distal end 21 side. Theripper shank 10 includes a body portion 12 including its proximal end 14and an insert portion 13 including its distal end 15 on the side to beinserted into the recess 22. The recess 22 formed in the ripper point 20has its bottom region 22A not in contact with the distal end 15 of theripper shank 10. There is a space 29 between the bottom region 22A ofthe recess 22 and the distal end 15.

In the ripper device 1 in the present embodiment, the ripper shank 10 asthe impact and wear resistant component is made of a steel containingnot less than 0.41 mass % and not more than 0.44 mass % C, not less than0.2 mass % and not more than 0.5 mass % Si, not less than 0.2 mass % andnot more than 1.5 mass % Mn, not less than 0.0005 mass % and not morethan 0.0050 mass % S, not less than 0.6 mass % and not more than 2.0mass % Ni, not less than 0.7 mass % and not more than 1.5 mass % Cr, notless than 0.1 mass % and not more than 0.6 mass % Mo, not less than 0.02mass % and not more than 0.03 mass % Nb, not less than 0.01 mass % andnot more than 0.04 mass % Ti, not less than 0.0005 mass % and not morethan 0.0030 mass % B, and not less than 20 mass ppm and not more than 60mass ppm N, with the balance consisting of iron and unavoidableimpurities, and having a hardness of HRC 53 or more and HRC 57 or less.The steel includes a matrix including a martensite phase and a residualaustenite phase, and first nonmetallic particles dispersed in the matrixand including at least one species selected from the group consisting ofMnS, TiCN, and NbCN. The steel does not include a carbide represented asM₂₃C₆ (where M represents the metallic elements constituting the steel).The amount of the residual austenite included in the matrix is 10 vol %or less, for example, and preferably 5 vol % or less.

The steel constituting the ripper shank 10 may further contain at leastone species selected from the group consisting of not less than 0.05mass % and not more than 0.20 mass % V, not less than 0.01 mass % andnot more than 0.15 mass % Zr, and not less than 0.1 mass % and not morethan 2.0 mass % Co.

The ripper shank 10 as the impact and wear resistant component of thepresent embodiment adopts the steel having the above-describedappropriate component composition as the material, and the steelstructure does not include M₂₃C₆ carbides. Accordingly, the ripper shank10 as the impact and wear resistant component of the present embodimentis an impact and wear resistant component excellent in durability.

In the ripper shank 10, the matrix of the steel constituting the rippershank 10 preferably has the grain size number (ASTM) of 5 or more and 8or less. This facilitates imparting excellent toughness to the rippershank 10.

In the ripper shank 10, the martensite phase constituting the matrix ofthe steel is preferably a low temperature-tempered martensite phase.This facilitates imparting excellent toughness to the ripper shank 10.

An exemplary method of producing a ripper shank 10 as the impact andwear resistant component of the present embodiment will now be describedwith reference to FIG. 4. In the method of producing the ripper shank 10in the present embodiment, firstly, a steel material preparing step isperformed as a step S10. In the step S10, a steel material made of thesteel having the above-described appropriate component composition isprepared.

Next, a hot working step is performed as a step S20. In the step S20,the steel material prepared in the step S10 is subjected to hot forgingor hot rolling and forming processing. With this, a formed body havingan approximate shape of the ripper shank 10 is obtained. Hot forging orhot rolling is performed by, for example, heating the steel materialprepared in the step S10 to a temperature not lower than 1200° C., suchas 1250° C. In the cooling process following the hot forging or hotrolling, M₂₃C₆ carbides are formed at the grain boundaries of the steel.

Next, a normalizing step is performed as a step S30. In the step S30,the formed body obtained in the step S20 is subjected to normalizingtreatment. Specifically, the formed body is firstly heated to atemperature range of not lower than 945° C. and not higher than 1000°C., and then cooled from the temperature range to a temperature nothigher than the temperature corresponding to the M_(s) point of thesteel. In this manner, the entirety of the formed body is normalized.Performing the normalizing treatment of heating to the temperature rangeof 945° C. or higher and 1000° C. or lower and then cooling causes theM₂₃C₆ carbides generated in the step S20 to dissolve into the matrix ofthe steel and disappear.

Next, a hardening treatment step is performed as a step S40. In the stepS40, the formed body having undergone the normalizing treatment in thestep S30 is firstly heated to a temperature range of 840° C. or higherand 920° C. or lower, for example, and then cooled from the temperaturerange to a temperature not higher than the M_(s) point of the steel. Inthis manner, the entirety of the formed body is quench hardened. Thecooling to the temperature not higher than the M_(s) point of the steelcan be performed, for example, by water cooling or oil cooling adoptingwater or oil as a cooling medium. The water cooling or oil cooling iscontinued until, for example, the surface temperature of the formed bodybecomes a temperature not lower than 50° C. and not higher than 100° C.Thereafter, the formed body is heated to a temperature range of notlower than 150° C. and not higher than 250° C. and then cooled to a roomtemperature (low temperature tempering). With this, the hardness of thesteel constituting the formed body is adjusted to a range of HRC 53 ormore and HRC 57 or less.

Next, a finishing step is performed as a step S50 as required. In thestep S50, the formed body obtained through the steps S10 to S40 issubjected to any necessary finishing or other treatment. The rippershank 10 in the present embodiment can be produced through theabove-described process. The obtained ripper shank 10 is combined with aseparately prepared ripper point 20, to obtain a ripper device 1.

According to the method for producing the ripper shank 10 of the presentembodiment, the M₂₃C₆ carbides, generated along the grain boundaries ofthe steel during hot forging or hot rolling and forming the steelmaterial made of the steel having the above-described appropriatecomponent composition, are made to disappear by the normalizingtreatment in the step S30, before the hardening treatment in the stepS40. In this manner, the ripper shank 10 as the impact and wearresistant component excellent in durability can be produced.

Examples

Samples corresponding to the impact and wear resistant component of thepresent application were prepared using four types of steel materials,including one made of a steel having the above-described appropriatecomponent composition, and experiments for evaluating their propertieswere conducted. The experimental procedures were as follows.

Table 1 shows chemical compositions of the steels used in theexperiments. The values in Table 1 are in mass %. The steel material Ahas a component composition corresponding to the steel constituting theimpact and wear resistant component of the present invention (InventiveExample). The steel materials B, C, and D have component compositionsfalling outside the scope of the present invention (ComparativeExamples). The steel materials B, C, and D correspond to SCrB430H, JISstandard SNCM431H, and the steel disclosed in the aforementioned PatentLiterature 1, respectively.

TABLE 1 C Si Mn P S Ni Cr Mo Nb Ti Al B N Fe A 0.43 0.30 0.40 0.0080.004 1.29 0.99 0.48 0.03 0.02 0.033 0.0024 0.0035 Bal. B 0.30 0.23 0.930.021 0.015 0.05 1.09 0.03 not not 0.030 0.0017 not Bal. measuredmeasured measured C 0.34 0.17 0.68 0.017 0.007 1.62 0.73 0.18 not not0.028 not not Bal. measured measured measured measured D 0.41 0.30 0.470.010 0.007 0.03 0.96 0.50 0.03 0.02 0.044 0.0022 0.0051 Bal.

(Experiments on Mechanical Properties)

The steel materials in Table 1 were used to prepare samples through aprocess similar to the steps S10 to S40 in the above embodiment. Fromthe obtained samples, tensile test specimens and Charpy impact testspecimens (2 mm U-notch) were produced, and a tensile test, an impacttest, and a Rockwell hardness measurement were conducted.

For the steel material A (Inventive Example) alone, the amount ofresidual austenite was measured using an X ray. The test results areshown in Table 2.

TABLE 2 0.2% Proof Tensile Reduction Impact Residual Stress StrengthElongation of Area Value Hardness γ Amount (MPa) (MPa) (%) (%) (J/cm²)(HRC) (vol %) A 1592 2131 14 44 62 56 2.3 B 1417 1655 13 47 65 48 notmeasured C 1414 1752 15 44 60 50 not measured D 1599 1935 13 43 59 53not measured

Referring to Table 2, when comparing the Inventive Example with theComparative Examples, the Inventive Example has achieved high values forthe 0.2% proof stress, tensile strength, and impact value, whilemaintaining the reduction of area comparable to those of the ComparativeExamples. Further, for the steel material A as the Inventive Example, ascompared to the steel material D, the tensile strength has improvedconsiderably despite their comparable 0.2% proof stress. The abovedemonstrates that the impact and wear resistant component of the presentapplication is excellent in durability.

(Experiment on Steel Structure)

The steel material A in Table 1 (the steel material corresponding to theexample of the present invention) was used to prepare a sample of aripper shank in a similar procedure as in the above embodiment. A testspecimen was taken from the sample. The surface of the obtained testspecimen was polished and then etched with a nitric acid alcoholsolution, and a microstructure was observed using an optical microscope.FIG. 5 shows optical micrographs showing the microstructure of thesteel.

Referring to FIG. 5, it can be seen from the microstructure of the steelthat the matrix includes a low temperature-tempered martensite phase. Inthe impact and wear resistant component of the present application, thepresence of some residual austenite (of 10 vol % or less) is acceptable.For the sample obtained in a similar manner, the amount of residualaustenite was measured using an X ray, and it was found that theresidual austenite of 1 vol % to 3 vol % was present. The abovedemonstrates that the matrix of the steel includes the martensite phaseand the residual austenite phase.

FIG. 6 shows photographs indicating the results of analysis by energydispersive X-ray spectroscopy (EDX) of products that were found throughobservation of the steel structure with SEM. As shown in FIG. 6, it isconfirmed that nonmetallic particles having a size of about 1 μm toabout 20 μm (first nonmetallic particles including at least one speciesselected from the group consisting of MnS, TiCN, and NbCN) are dispersedin the matrix of the steel.

(Experiments on Carbides Formed at Grain Boundaries)

The steel material A (the steel material corresponding to the example ofthe present invention) in Table 1 was used to prepare a test specimen(as quenched; sample A) by performing the process of the aboveembodiment up to the step S20 (with the forging temperature of 1250°C.), not performing the step S30, and performing quenching treatment inthe step S40 after heating the material to 870° C. A test specimen (asquenched; sample B) was also prepared, by similarly performing theprocess up to the step S20, performing normalizing treatment in the stepS30 by heating the material to 970° C., and further performing quenchingtreatment in the step S40 after heating the material to 870° C. For thesamples A and B, the microstructures were observed with an opticalmicroscope and SEM, and for products present along the grain boundaries,elemental mapping was conducted with EDX. The experimental results areshown in FIG. 7.

Referring to FIG. 7, it can be seen that carbides of Mo and Cr arepresent along the grain boundaries in the sample A for which the stepS30 was omitted, and that a bainite structure is formed around thecarbides. The formation of the bainite structure is conceivablyattributable to the local decrease in the amount of alloy elementsbecause of the formation of the above carbides, and the resultantreduction in hardenability. In contrast, in the sample B correspondingto the impact and wear resistant component of the present invention forwhich normalizing treatment was conducted in the step S30 with theheating temperature of 970° C., no carbides as described above werefound. The above experimental results show that although theabove-described carbides formed during the hot working process remainwith the quenching temperature of 870° C., the carbides dissolve anddisappear with the normalizing temperature of 970° C.

An example of the identification of carbides present in the sample A isshown in FIG. 8, in which a carbide was detected in a bright-field imageof STEM and then the selected area diffraction (SAD) pattern of thecarbide was confirmed. As shown in FIG. 8, it can be seen that thecarbide is a M₂₃C₆ carbide. That is to say, it has been confirmed thatin the method of producing an impact and wear resistant component of thepresent application, the M₂₃C₆ carbides formed during the hot workingprocess disappear by the heating during the normalizing conducted in thestep S30.

(Experiment on Relationship between Heating Temperature and Reduction ofArea)

The steel material A in Table 1 was used to prepare test specimens whichwere quench hardened by rapid cooling from various temperatures and thentempered at high temperature. The test specimens were subjected to atensile test. At this time, the heating temperature upon quenching wasvaried to investigate the effect of the heating temperature on thereduction of area in the tensile test. The experimental results areshown in FIG. 9.

Referring to FIG. 9, it can be seen that the reduction of area clearlyincreases with the heating temperature of 945° C. or higher. Thistemperature range of not lower than 945° C. agrees with the temperaturerange in which M₂₃C₆ carbides cease to be seen in the experiment on thecarbides formed at the grain boundaries. This indicates that the M₂₃C₆carbides generated at the grain boundaries of the steel can beeliminated by the heating to the temperature range of not lower than945° C., whereby the reduction of area is improved.

While the ripper shank was described as an example of the impact andwear resistant component of the present application in the aboveembodiment, the impact and wear resistant component of the presentapplication is applicable to a variety of impact and wear resistantcomponents made of a steel having a hardness of HRC 53 or more and HRC57 or less, such as bucket teeth, bucket adapters, bucket shrouds,ripper points, protectors, cutting edges, end bits, crusher teeth,sprocket teeth, springs, shoe plates, shoe bolts, and the like.

It should be understood that the embodiment and examples disclosedherein are illustrative and non-restrictive in every respect. The scopeof the present invention is defined by the terms of the claims, ratherthan the description above, and is intended to include any modificationswithin the scope and meaning equivalent to the terms of the claims.

REFERENCE SIGNS LIST

1: ripper device; 10: ripper shank; 11: ripper shank through hole; 12:body portion; 13: insert portion; 14: proximal end; 15: distal end; 20:ripper point; 21: distal end; 22: recess; 22A: bottom region; 23:proximal end; 25: ripper point through hole; 29: space; 31: arm; 32:lift cylinder; 33: tilt cylinder; 34: ripper support member; and 51:pin.

1. An impact and wear resistant component made of a steel containing notless than 0.41 mass % and not more than 0.44 mass % C, not less than 0.2mass % and not more than 0.5 mass % Si, not less than 0.2 mass % and notmore than 1.5 mass % Mn, not less than 0.0005 mass % and not more than0.0050 mass % S, not less than 0.6 mass % and not more than 2.0 mass %Ni, not less than 0.7 mass % and not more than 1.5 mass % Cr, not lessthan 0.1 mass % and not more than 0.6 mass % Mo, not less than 0.02 mass% and not more than 0.03 mass % Nb, not less than 0.01 mass % and notmore than 0.04 mass % Ti, not less than 0.0005 mass % and not more than0.0030 mass % B, and not less than 20 mass ppm and not more than 60 massppm N, with the balance consisting of iron and unavoidable impurities,and having a hardness of HRC 53 or more and HRC 57 or less, the steelincluding a matrix including a martensite phase and a residual austenitephase, and first nonmetallic particles dispersed in the matrix andincluding at least one species selected from the group consisting ofMnS, TiCN, and NbCN, the steel not including a carbide represented asM₂₃C₆ (where M represents the metallic elements constituting the steel).2. The impact and wear resistant component according to claim 1, whereinthe steel further contains at least one species selected from the groupconsisting of not less than 0.05 mass % and not more than 0.20 mass % V,not less than 0.01 mass % and not more than 0.15 mass % Zr, and not lessthan 0.1 mass % and not more than 2.0 mass % Co.
 3. The impact and wearresistant component according to claim 1, wherein the matrix has a grainsize number of 5 or more and 8 or less.
 4. The impact and wear resistantcomponent according to claim 1, wherein the martensite phaseconstituting the matrix is a low temperature-tempered martensite phase.5. A method for producing an impact and wear resistant component,comprising the steps of: preparing a steel material made of a steelcontaining not less than 0.41 mass % and not more than 0.44 mass % C,not less than 0.2 mass % and not more than 0.5 mass % Si, not less than0.2 mass % and not more than 1.5 mass % Mn, not less than 0.0005 mass %and not more than 0.0050 mass % S, not less than 0.6 mass % and not morethan 2.0 mass % Ni, not less than 0.7 mass % and not more than 1.5 mass% Cr, not less than 0.1 mass % and not more than 0.6 mass % Mo, not lessthan 0.02 mass % and not more than 0.03 mass % Nb, not less than 0.01mass % and not more than 0.04 mass % Ti, not less than 0.0005 mass % andnot more than 0.0030 mass % B, and not less than 20 mass ppm and notmore than 60 mass ppm N, with the balance consisting of iron andunavoidable impurities; hot forging or hot rolling the steel material toobtain a formed body; performing normalizing treatment on an entirety ofthe formed body by cooling the formed body from a temperature not lowerthan 945° C. and not higher than 1000° C. to a temperature not higherthan a temperature corresponding to the M_(s) point of the steel; andperforming quench hardening treatment on the formed body havingundergone the normalizing treatment and, thereafter, adjusting ahardness of the formed body to be HRC 53 or more and HRC 57 or less byheating the formed body to a temperature not lower than 150° C. and nothigher than 250° C.
 6. The impact and wear resistant component producingmethod according to claim 5, wherein the steel further contains at leastone species selected from the group consisting of not less than 0.05mass % and not more than 0.20 mass % V, not less than 0.01 mass % andnot more than 0.15 mass % Zr, and not less than 0.1 mass % and not morethan 2.0 mass % Co.
 7. The impact and wear resistant component accordingto claim 2, wherein the matrix has a grain size number of 5 or more and8 or less.
 8. The impact and wear resistant component according to claim2, wherein the martensite phase constituting the matrix is a lowtemperature-tempered martensite phase.
 9. The impact and wear resistantcomponent according to claim 3, wherein the martensite phaseconstituting the matrix is a low temperature-tempered martensite phase.10. The impact and wear resistant component according to claim 7,wherein the martensite phase constituting the matrix is a lowtemperature-tempered martensite phase.